Electric discharge machine having a balance device for balancing gravitational force acting on a vertically moveable quill

ABSTRACT

An electric discharge machine having an electrode mounting device ( 42 ) for mounting a tool electrode, a quill ( 41 ) movable in the vertical direction and having the tool mounting device attached to a lower end, at least two linear motor movers ( 43, 44 ) attached to the quill and disposed symmetrically about the central axis of the quill, at least two linear motor stators ( 31, 32 ), each disposed opposite one of the at least two movers, and a balance device ( 6 ) for balancing a gravitational force acting on the quill. The balance device has an air cylinder ( 60 ) arranged coaxially with the quill so that a balance force having a force vector aligned with the central axis of the quill is generated. The air cylinder is preferably mounted in a hole formed in the center of the quill and extends in a vertical direction.

FIELD OF THE INVENTION

The present invention relates to an electric discharge machine forforming holes of desired shapes in a workpiece while repeatedly causingelectric discharges to occur between a tool electrode and a conductiveworkpiece, while moving the tool electrode towards the workpiece. Moreparticularly, the present invention relates to an electric dischargemachine having a balance device for balancing gravitational force actingon a vertically moveable quill.

BACKGROUND OF THE INVENTION

Electric discharge machines are widely used to accurately process solidconductive workpieces into molds and dies. The workpiece is fixed to atable arranged in a work tank, and a tool electrode of copper orgraphite is attached to a quill, which is movable in a verticaldirection, using a suitable electrode mounting device. The work tank isfilled with dielectric fluid, such as kerosene, and the tool electrodeis positioned extremely close to the workpiece. The space between theworkpiece and the tool electrode is referred to as a “gap”, also knownas the “machining gap”; the size of this gap ranges from a few μm to afew tens of μm. By applying a power pulse across the workpiece and thetool electrode during a controlled pulse ON time, the insulationcharacteristics of the dielectric fluid in the gap are broken down and adischarge is produced. Microscopic amounts of the workpiece material areevaporated or made molten by the heat caused by this discharge, and flowinto the dielectric fluid. When the pulse ON time is completed, i.e.,during the pulse OFF time, the insulation characteristics of thedielectric in the gap are restored.

As a result of the discharges produced during the pulse ON time,microscopic crater shaped holes remain on the surface of the workpiece.At the completion of the pulse ON time, a controlled OFF time startsduring which application of a power pulse to the workpiece isinterrupted and the insulation characteristics of the dielectric fluidin the gap may be restored. Electric discharge machines usually have theON time and OFF time controlled in a range of between about 1 μsec and afew tens of msecs, to thereby apply a train of power pulses to the gap.

During machining the electric discharge machine causes the toolelectrode to move toward the workpiece, along the Z axis, in order tomaintain the gap at a relatively fixed size. As tool electrode advances,removing microscopic amounts of material from the workpiece withoutcontacting the workpiece, cavaties. complementary in shape to the toolelectrode are accurately formed in the workpiece. The cavities have goodsurface roughness characteristics. This type of electric dischargemachine, known as a sinker EDM is classified differently than a wireEDM, i.e., a machine which uses a moving wire electrode.

Use of an appropriate flushing procedure to produce a flow of dielectricfluid through the gap to wash fragments removed from the workpiece awayfrom the gap is important in electric discharge machines. The flushingprocedure serves to prevent undesired secondary discharges between thetool electrode and fragments that have been removed from the workpiece,and to restore reliable insulation during the pulse OFF time. A skilledoperator may form holes at appropriate positions in the sinker toolelectrode and the workpiece (before processing) in order to supply freshdielectric into the gap and to suck contaminated dielectric fluid out ofthe gap. When the shape of these machining fluid circulation holes islimited due to the size and shape of the tool electrode, the operatormay arrange an injection apparatus for injecting dielectric fluidtowards the gap at an appropriate position. Flushing is crucial tocarrying out good discharge machining faster and more accurately, butskill is necessary in order to produce a uniform flow throughout theentire gap. Causing the tool electrode to move periodically along the Zaxis, first to rapidly rise up and then to rapidly fall, so that thecontaminated dielectric fluid in the gap is almost completely removedfrom the cavities in the workpiece, is known as a “jump” operation.During a conventional jump operation, the tool electrode moves at a rateof a few hundred mm/min. As the reciprocating distance of the toolelectrode is made larger, more fresh fluid flows into the gap and morecontaminated fluid is ejected from the gap. Therefore, the toolelectrode is preferably caused to rise up during a jump operation to atleast the depth of a cavity being formed in the workpiece. However,since there is no removal of material from the workpiece during a jumpoperation, if jump operations are carried out too frequently, it resultsin a lowering of the stock removal rate. In order to perform a jumpoperation having a sufficiently large amount of movement withoutlowering the stock removal rate, the tool electrode is preferably movedwith an acceleration and deceleration in excess of 1 G.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an electric dischargemachine that has, in addition to a quill movable in the verticaldirection with high acceleration, a balancing device for balancinggravitational forces acting on the vertically moveable quill.

Another object of the present invention is to provide an electricdischarge machine that causes a quill, having a tool mounting electrodeattached to a lower end thereof, to move accurately in the verticaldirection with high acceleration.

Additional objects, advantages and novel features of the invention willbe set forth in the description that follows, and will become apparentto those skilled in the art upon reading this description or practicingthe invention. The objects and advantages of the invention may berealized and attained by practicing the invention as recited in theappended claims.

In order to achieve the above and other objects, there is provided anelectric discharge machine (EDM) according to the present invention formachining a workpiece by moving a tool electrode vertically towards aworkpiece while repeatedly producing electric discharges between theworkpiece and the tool electrode. The EDM comprises a vertically movablequill, an electrode mounting device attached to a lower end of the quillfor mounting the tool electrode, at least two linear motor moversattached to the quill symmetrically about the central axis of the quill,at least two linear motor stators, respectively opposite the at leasttwo movers, and a balance device for balancing a gravitational forceacting on the quill so that a line of action of a balance force can bealigned with the central axis of the quill.

Preferably, the balance device comprises an air cylinder arrangedcoaxially with the quill.

More preferably, the quill has a hole extending vertically in the centerand the air cylinder is located in the hole.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view illustrating a first embodiment of an electricmachining discharge apparatus of the present invention.

FIG. 2 is a perspective drawing illustrating the electric dischargemachining apparatus of FIG. 1.

FIG. 3 is an air circuit diagram of a balance device in FIG. 1.

FIG. 4 is a cross sectional drawing illustrating a brake assembly inFIG. 1.

FIG. 5 is a drawing showing the brake assembly of FIG. 1.

FIG. 6 is a side view illustrating a second embodiment of an electricmachining discharge apparatus of the present invention.

FIG. 7 is a perspective view of the electric discharge machiningapparatus in FIG. 6.

FIG. 8 is a side view illustrating a third embodiment of an electricmachining discharge apparatus of the present invention.

FIG. 9 is a perspective view of the electric discharge machiningapparatus in FIG. 8.

FIG. 10 is an air circuit diagram of a balance device in FIG. 9.

PREFERRED EMBODIMENT OF THE INVENTION

An example of an electric discharge machine 1 according to the presentinvention will now be described with reference to FIG. 1, FIG. 2, FIG. 3and FIG. 4.

As shown in FIG. 1, an electrode mounting device 42 (not shown in thedrawings) to which a tool electrode is attached, is fixed to a lower endof a quill 41 and arranged coaxially with the quill 41. The quill 41,which preferably has a square columnar shape, also preferably has acylindrical hole extending vertically through its center. The quill 41is movable at high speed in the vertical direction along the Z axis bymeans of two linear motors. In the illustrated embodiment, a linearmotor mover (rotor) comprises permanent magnets 44 and a magnetic plate43, while a linear motor stator comprises a yoke 31 and an armature coil32. The magnetic plate 43, on which a single row of permanent magnets 44are arranged, is fixed to parallel side walls opposite to the quill 41and is disposed symmetrically with respect to the permanent magnets 44.The permanent magnets 44 are respectively fastened to the magnetic plate43 and are preferably inclined slightly from the horizontal direction soas to cause a reduction in torque ripple. A column 1 of the electricdischarge machining apparatus is located above a bed (not shown) and aframe 2 is attached to the column 1. The yoke 31 around which thearmature coil 32 is wound is fixed to the frame 2 so as to be oppositethe row of permanent magnets 44. The gap between the mover and thestator is maintained at such a size that a center of thrust generated atboth side surfaces opposite to the quill 41 is aligned with the centralaxis of the quill 41. A coolant pipeline 33 is formed in the yoke 31. Ifcurrent flows in the armature coil 32, the quill 41 moves in thevertical direction by means of the force generated between the mover andthe stator. Stoppers 45 and 46 for determining the limit of travel ofthe quill 41 are respectively provided on upper and lower ends of thequill 41. The stoppers 45 and 46 preferably contain resilient membersfor absorbing shock when they respectively collide with the frame 2. Thequill 41 is guided by a guide assembly comprising a guide rail 52 and apair of linear motor roller bearings 51. The guide rail 52 is providedon the inner surface of the quill 41 that is not provided with themover. The pair of linear motor roller bearings 51 are provided on theframe 2 and engage with the guide rail 52.

A balance mechanism 6 for balancing a load of a moving body capable ofmoving in the vertical direction at a speed in excess of 1 G comprisesan air cylinder 60 and an air supply 61. The air supply 61 contains acylinder 62 located in a hole of the quill 41 coaxial with the quill 41,a piston 63 capable of reciprocally sliding inside the cylinder, and apiston rod 64 having one end linked to the piston 63. A flange 65 of thecylinder 62 is rigidly fixed to an upper end of the quill 41 by bolts.The piston rod 64 is supported at its other end by a bracket 7. Thebracket 7 comprises a top plate 71 fixed to the other end of the pistonrod 64 and a side wall 72 for supporting the top plate 71. The side wall72 is fixed to the frame 2. Because the cylinder 62 is provided directlynext to the quill 8, the air cylinder 60 very responsively tracksmovement of the quill 41.

The air supply 61 will now be described in detail with reference to FIG.3.

A supply port 66 for either supplying air to or discharging air from anupper chamber 68 formed inside the cylinder 62 at a location higher thanthe piston 63. The amount of air inside the upper chamber 68 iscontrolled in response to the position of the quill 41 so as to maintainthe air pressure inside the upper chamber 68 at a fixed value. A lowerchamber 69 is formed in the cylinder 62 at a location lower down thanthe piston 63, communicates with a silencer 67 and opens out to theatmosphere. The air supply 61 controls the air cylinder 60 to maintainthe pressure of compressed air supplied from a compressed air supply618, such as a compressor, at a set value. The air supply 61 contains afilter 611 for removing dirt and dust from the compressed air, a filter612 for further removing oil mist from the compressed air, and an airregulator 613 for adjusting the compressed air to be supplied at aspecified pressure. The air supply 61 also has a high relief airregulator 614 for maintaining the air pressure inside the upper chamber68 at the set value by supplying and discharging air to and from theupper chamber 68 at high speed. The high relief air regulator 614 mayactually comprise a large capacity air regulator provided with a largecapacity relief valve. If the air pressure inside the upper chamber 68becomes higher than the set value due to lowering of the quill 41, thehigh relief air regulator 614 opens the relief valve and discharges airinside the upper chamber 68 from the supply port 66 at high speed. Ifthe air pressure inside the upper chamber 68 becomes lower than the setvalue due to raising of the quill 41, the high relief air regulator 614supplies air into the upper chamber 68 from the supply port 66 at highspeed with the relief valve closed. The air pressure inside the upperchamber 68 is set so as to generate a balance force to counteract thegravitational force acting on the quill 41 primarily due to the mass ofthe quill 41, the linear motor mover, the electrode mounting device 42and the tool electrode. This set value can be easily varied using, e.g.,a button (not shown) provided on the high relief air regulator 614. Theair pressure is set depending on the weight of the tool electrodeattached to the machine, for example in the range 2.5-6 kg/cm². To makeit easy for the operator to set the air pressure, the air pressure scalepreferably indicates the weight of the tool electrode.

As a result of the balance device 6, the weight of the moving body,primarily including the quill 41, linear motor movers, electrodemounting device and tool electrode, does not impose any load on thelinear motor, and therefore electric power is conserved while the quill41 is stationary. Also, the balance device 6 reduces the output of thelinear motor, which means that generation of heat from the linear motoris minimized and the mover of the linear motor may be made lightweight.Making the moving body lightweight contributes to the high overallacceleration of the moving body. Since the cylinder 62 and the quill 41are arranged coaxially, a line of action of a balance force generated inthe air cylinder 60 is aligned with the central axis of the quill 41.The line of action of the balance force is also aligned with the centerof thrust generated by the linear motor. Accordingly, no bending momentacts on the quill 41 or the air cylinder 61, thereby ensuring the quill41 will move in a straight line.

A brake assembly is provided on the flange 65 for preventing theworkpiece and the tool electrode from colliding if the tool electrodeunexpectedly drops. As shown in FIG. 4, a push member 83 opens a pair oflevers 84 using compressed air supplied to the brake assembly 8 from aport 82. In this state, the levers 84 and a pair of integral brake shoes86 oppose the resilient force of an armature coil spring 85, to open apiston rod 64 penetrating through the brake assembly 8. As shown in FIG.5, compressed air is conveyed from the compressed air supply 618,through an electromagnetic valve 81 to the brake assembly 8.

Referring to FIG. 5, operation of the brake assembly 8 when anelectricity failure occurs will be described. Reference characters ELand WP respectively represent a tool electrode and a workpiece. Anuninterruptible power supply 91, e.g., a battery supplies electricalpower to a controller 615, a motor driver 10, and a relay circuit 616 assoon as an electricity failure is detected. The uninterruptible powersupply 91 is continuously charged using an a/c power source 9. If asignal indicating an electricity failure is received from theuninterruptible power supply 91, the controller 615 transmits a signalto the motor driver 10 to cause the quill 41 to be raised or to belowered by a specified distance. The controller 615 also transmits asignal causing compressed air to be discharged from the brake assembly 8to the relay circuit 616 for the electromagnetic valve 81. Theelectromagnetic valve 81 blocks the air passage between the compressedair supply 618 and the brake assembly 8, and discharges compressed airfrom the port 82 of the brake assembly 8. If the resilient force of thearmature coil spring 85 overcomes the air pressure inside the brakeassembly, the pair of brake shoes 86 are pressed against the piston rod64. Thereby, the quill 41 is prevented from falling, even if there is anelectric power failure.

The brake assembly will operate even in the event that the pressure ofthe air supplied to the air cylinder 60 is abnormally low. Abnormallowering of air pressure is detected by a pressure switch 617 providedbetween the air regulator 613 and the air cylinder 60. If the controller615 receives a detection signal from the pressure switch 617, a warningis issued and the brake assembly is caused to operate in the same way asfor an electricity failure.

Referring to FIG. 6 and FIG. 7, a second embodiment of an electricdischarge machining apparatus of the present invention will now bedescribed. Elements that are the same as those in FIG. 1, FIG. 2, FIG. 3FIG. 4 and FIG. 5 have the same reference numerals attached thereto, anddescription thereof will be omitted.

The brake assembly 8 is fixed to a frame 2 between a column 1 and aquill 41. A shaft 87, moveable in a direction parallel to a piston rod64, passes through the brake assembly 8. The movable shaft 87 is guidedby a bearing 88 inserted into through holes of the frame 2. An upper endof the movable shaft 87 is connected to a upper end of the quill 41 by ahorizontally extending bridge plate 73. A side wall 72 of a bracket 7has an opening communicating with the bridge plate 73. In thisembodiment, a brake assembly 8 is located at an upper section of thequill 41, which means that the height of a head section of the machinecan be made small.

Referring to FIG. 8 and FIG. 9, a third embodiment of an electricdischarge machining apparatus of the present invention will now bedescribed. Elements that are the same as those in FIG. 1, FIG. 2, FIG. 3FIG. 4 and FIG. 5 have the same reference numerals associated therewith,and description thereof will be omitted.

Differing from the first and second embodiments, magnetic plates 43 arerespectively fixed to quill side walls parallel to vertical surfaces 1 aof a coil 1. However, similarly to the other embodiments, two linearmotors are arranged so that the thrust center of the motors is alignedwith the central axis of the quill 41. A flange 65 of an air cylinder 60is rigidly fixed to a frame 2 with bolts so that a cylinder 62 isarranged parallel to the quill 41 between the quill 41 and a column 1. Alower end of a support rod 74 extending coaxially with the quill 41 isfixed to an upper end of the quill 41. An upper end of a piston rod 64is connected to an upper end of the support rod 74 using a bridge plate73 extending horizontally. In applications where it is most important tomake the moving body, including the quill 41, light in weight, the aircylinder 60 and the brake assembly 8 may be removed from the moving bodyin this way.

As shown in FIG. 10, air may be supplied either from a supply port 66 toa lower chamber 69 formed inside the cylinder 62 lower down than apiston 63, or discharged from the lower chamber 69. The amount of airinside the lower chamber 69 is controlled according to the position ofthe quill 41 so that the air pressure inside the lower chamber 69 ismaintained at a specified value. An upper chamber 68 formed in thecylinder 62 lower than the piston 63 communicates with a silencer 67 andopens to the atmosphere.

The foregoing description of preferred embodiments of the invention havebeen presented for the purposes of illustration and description. Theyare not intended to be exhaustive or to limit the invention to theprecise forms disclosed, and obviously various modifications andvariations are possible in light of the above teachings. It is intendedthat the scope of the invention be defined by the appended claims.

What is claimed is:
 1. An electric discharge machining apparatus formachining a workpiece by moving a tool electrode towards a workpiece ina vertical direction while causing electric discharge between theworkpiece and the tool electrode, comprising: a quill movable in thevertical direction; an electrode mounting device for mounting the toolelectrode, attached to a lower end of the quill; at least two linearmotor movers attached to the quill symmetrical about the central axis ofthe quill; at least two linear motor stators, respectively opposite theat least two movers; and a balance device for balancing a gravitationalforce acting on the quill so that a line of action of a balance force isaligned with the central axis of the quill.
 2. The electric dischargemachining apparatus of claim 1, wherein the balance device comprises anair cylinder arranged coaxially with the quill.
 3. The electricdischarge machining apparatus of claim 2, wherein the quill has a holeextending vertically in the center, and the air cylinder is located inthis hole.
 4. The electric discharge machining apparatus of claim 1,wherein the balance device comprises a cylinder head fixed to andcoaxial with the quill, a piston capable of reciprocating inside thecylinder, and a fixed piston rod connected to the piston.
 5. Theelectric discharge machining apparatus of claim 4, further comprising abrake device for applying a braking force to the piston rod.
 6. Thedevice according to claim 5 wherein said brake device comprises a pairof brake shoes disposed about said piston rod and a pair of actuatorsfor said brake shoes, and further comprising a controller, responsive toa loss of power condition to operate said actuators to operate saidbrake shoe to bear against said piston rod.
 7. The device according toclaim 6 wherein said actuators comprise a pair of levers responsive to apneumatically operated member to open and close said brake shoes.
 8. Thedevice according to claim 7 further comprising a pneumatic circuitincluding a source of air and an electromagnetic valve, saidelectromagnetic valve being responsive to a loss of power condition toadmit air from said source to operate said pneumatic member.
 9. Theelectric discharge machining apparatus of claim 4, comprising a movableshaft extending parallel to the piston rod, a bridging plate extendinghorizontally for connecting the movable shaft to the quill, and a brakedevice for applying a braking force to the movable shaft.
 10. Theapparatus according to claim 4 wherein said piston defines a boundarybetween an upper chamber of said cylinder and a lower chamber and theamount of air in the upper chamber is controlled in response to quillposition so as to maintain the pressure in said upper chamber at agenerally fixed value.
 11. The apparatus according to claim 10 furthercomprising a high speed relief valve connected to said upper chamber forsupplying air to and discharging air from said upper chamber at highspeed in response to movement of said quill.
 12. The electric dischargemachining apparatus of claim 1, wherein the tool mounting device isarranged coaxially with the quill.
 13. An electric discharge machiningapparatus for machining a workpiece by moving a tool electrode towards aworkpiece in a vertical direction while causing electric dischargebetween the workpiece and the tool electrode, comprising: a quillmovable in the vertical direction; an electrode mounting device formounting the tool electrode, attached to a lower end of the quill; atleast two linear motor movers attached to the quill symmetrical aboutthe central axis of the quill; at least two linear motor stators,respectively opposite the at least two movers; a cylinder fixed parallelto the quill; a piston capable of reciprocating inside the cylinder; apiston rod connected to the piston; a support rod fixed to the quill andextending coaxially with the quill; and a bridging plate extendinghorizontally for connecting the piston rod and the support rod.
 14. Theapparatus according to claim 13 wherein said bridge plate extendshorizontally between the piston rod and the support rod.
 15. Theapparatus according to claim 13 wherein said piston defines a boundarybetween an upper chamber of said cylinder and a lower chamber and theamount of air in the upper chamber is controlled in response to quillposition so as to maintain the pressure in said upper chamber at agenerally fixed value.
 16. The apparatus according to claim 15 furthercomprising a high speed relief valve connected to said upper chamber forsupplying air to and discharging air from said upper chamber at highspeed in response to movement of said quill.